The invention relates to a method and a facility for cooling metal parts that have been subjected to a molten salt bath nitriding/nitrocarburizing treatment. The invention also relates to the parts so treated.
The use of methods that employ a thermochemical diffusion of nitrogen by nitriding or nitrocarburizing in baths of molten salts to reduce the coefficient of friction and improve the adhesive and abrasive wear resistance of metal parts is fully understood by those skilled in the art. In the main, these salt baths generally comprise cyanate and alkaline carbonate. When the nitriding temperature is reached, the alkaline cyanate releases nitrogen and carbon which diffuse over the surface of the part. The treatment times are generally between 20 and 180 mn at temperatures of between 400 and 700° C. These industrially used processes are known for example under the brand names SURSULF or TENIFER.
It will be remembered that a nitriding/nitrocarburizing treatment process comprises the following main steps:                degreasing the parts,        preheating,        nitrocarburizing treatment,        cooling,        rinsing,        drying.        
When ferrous alloys are concerned, this treatment generally causes two characteristic zones to form: a first surface zone, with a thickness of between 5 and 30 μm consisting mainly of ε nitrides (Fe2-3n) and γ′ nitrides (Fe4N), known as the compound zone, followed by a second zone, with a thickness generally of between 0.2 and 1.5 mm characterized by the presence of nitrogen in solid solution in the iron grains and in the form of nitrides of alloying elements, known as the diffusion layer.
Various alternative methods of cooling after the nitrocarburizing treatment have been developed in order to improve some features of the treated parts:                an improvement in the corrosion resistance of the treated parts is obtained by replacing water quench cooling by an oxidizing salt bath quench (380-420° C.). Treatment of this kind, known for example under the brand names Arcor® or AB1®, produces a black iron oxide (Fe3O4) on the treated surface.        a reduction in the brittleness, or an improvement in the ductility of the parts treated, is obtained by replacing water quench cooling by cooling that is slower such as oil cooling or slower still air cooling. Slow cooling is also recommended for parts that cannot withstand significant distortion. The parts obtained are characterized by the presence of iron nitride precipitates □′-Fe4N and α″-Fe16N2, parallel to the grain boundaries in the diffusion layer. The precipitation is related to the decrease in the solubility limit of nitrogen in iron with the temperature.        
For an industrial treatment of parts, the latter are placed in a metal rack to facilitate the transportation thereof, using robots for example, between the various treatment stations. For reasons of productivity, the fill factor of the rack is at maximum, so that the parts are able to be in contact with each other. The parts are transferred from the nitriding bath to the cooling zone for a length of time such that, in contact with the ambient air, oxidation or surface discolouration spots appear on the surface of a more or less significant portion of the treated parts. Tests carried out in the laboratory have shown that after a transfer time of more than about 30 seconds, oxidation spots are seen to appear on some parts only, whereas after a transfer time of some 120 seconds, all the parts are oxidized. As it happens, the industrial transfer time between two successive treatment zones is generally between these two values.
It should also be noted that air cooling inevitably induces a surface oxidation of the parts.
It is quite obvious that the presence of these oxidation spots is not acceptable for some applications. Not only are these spots detrimental to the appearance of the parts, but also to their use, particularly for applications that are exacting in terms of surface cleanness. Indeed, the oxidized zones generate dust which may, when lubricants are present, create aggregates, and bring about abrasive wear that is harmful for the intended application.
In the current state of the art, the industrial solutions proposed cannot ensure a molten salt bath nitriding/nitrocarburizing treatment that has a sufficiently high degree of cleanness and is of good enough appearance, in other words with no trace of oxidation on any of the parts treated.
In this respect, it should be recalled that the technical field of the invention relates to an industrial treatment of parts which cannot be compared with a nitriding/nitrocarburizing treatment performed at laboratory level where the parts are only treated in small quantities. As a result, in the laboratory, after the nitriding bath, the parts are able to be transferred quickly enough to avoid oxidation during water cooling for example.
It will be understood that this is not possible on an industrial scale, when a large number of parts is to be treated simultaneously, thereby generating a significant rejection rate. Even by reducing the part transfer time as much as possible, particularly between the treatment zone and the cooling zone, it then proves necessary to conduct a visual inspection and a unit sort out of the parts if the absence of oxidation traces is to be guaranteed.
U.S. Pat. No. 3,560,271 relates to a method of nitriding in baths of molten salts with the aim of slowing down cooling after nitriding so as to reduce the working stress levels in order thereby to limit the risk of the layer cracking Vacuum cooling can only occur through radiation, thereby giving cooling times that are not easily compatible with an industrial process (from several hours to several tens of hours).
Moreover, the use of said process does not ensure the complete absence of any trace of oxidation when treating a large number of parts which requires relatively high transfer times between the treatment station and the cooling station (i.e when transferring the loads, mass inertia compels part load stabilization phases after deceleration particularly during horizontal transfers, and therefore minimum transfer times).
It is clear therefore from an analysis of the prior art that the industrial solutions in use cannot ensure a molten salt bath nitriding/nitrocarburizing treatment that has a sufficiently high degree of cleanness and is of good enough appearance, in other words, with no trace of oxidation on any one portion or on any of the parts treated.
It will also be understood that it is not possible, especially where an industrial treatment is concerned, to obtain parts that are sufficiently ductile while at the same time showing no trace of oxidation.
The purpose set by the invention is to overcome these drawbacks in a straightforward, safe, efficient and rational manner.
The problem the invention therefore sets out to resolve is that of ensuring, in respect of an industrial treatment of metal parts that have been subjected to a molten salt bath nitriding/nitrocarburizing treatment, that there are no traces of oxidation-corrosion, so that the ductility thereof can be improved.